A molecular dynamics study of the long-time ice Ih surface dynamics

Molecular dynamics simulations of the ice Ih surface between 180 and 210 K showed that the dynamics of the water molecules in the top two to three bil ayers is substantially faster than that of the molecules in the lower (bulk ) bilayers. Within the simulation time of tens of nanoseconds. there is rap id exchange of molecules between these upper bilayers, but not between thes e and the lower bilayers. In-plane translation of the molecules in the top surface bilayer leads to rapid surface reconstruction, and a structure cons isting primarily of water heptagons, hexagons, and pentagons replaces the c rystal geometry. An Arrhenius analysis of the diffusion of molecules in the top bilayer yields a barrier for in-plane diffusion E-0 = 23.2 +/- 2.9 kJ mol(-1) and a preexponential factor D-0 = 0.77 cm(2) s(-1) (ln(D-0) = -0.26 +/- 1.99). The activation barrier, which is the same as the experimentally measured value, is similar to the energy required to break a hydrogen bond and supports a diffusion mechanism where water molecules move by repeated breaking and formation of hydrogen bonds. The similarity between the simula ted and experimental barrier heights supports an in-plane diffusion mechani sm where molecules must move to the upper bilayers before diffusion can occ ur.